Insulating material containing mica



March 26, 1940.. w. H. MILLER 2,195,254

INSULATING MATERIAL CONTAINING MICA Filed July 8, 1938 Inventor- Wil l iam H. Miller;

v His Att orneg.

Patented Mar.26,1 940 2,195,254

UNITED STATES, PATENT oer-ice 2.195.254 msum'rmo m'rsamr. con'rnmo MICA William Howard Miller, Fort Wayne, 1nd,,

signor to General Electric Company, g, commtion of New York Application July 8, 1938, Serial No. 218.192 50mm. (Cl. 154-215) The present invention relates to composite inthermore, products which volatilize from shellac sulation containing mica and has as its prin- I at high temperatures have a tendency to attack cipal object to provide laminated mica structures copper in electrical devices, particularly in c mand articles having improved physical and elecmutators, forming a conductive compound which- 1 ,1 -m frequently produces destructive short circuits.

In certain applications of electrically insulat- With shellac and alkyd resins as binders, coning substances it is important that the insulasiderable time is required in the fabricati n of tion not only have a high dielectric strength but the laminated articles due to the relatively slow also be capable of withstanding relatively high rate at which these substances cure or harden.

l0 temperatures and high tensile strss. These Other bonding agents for mica have been de- 10 properties are indispensable, for example, for the flcient in various respects. Some of these maintermediate insulation of commutators, slip terials decompose at temperatures encountered rings and the like. For such purposes so-called during operation of electrical machinery, others "pasted mica, that is mica flakes cemented become brittle, otherswill not stick to mica 16 together in stratified relation with a suitable effectively throughout the temperature range of binder, commonly has been used. service use, and so on.

It is well known to those engaged in the pro- In accordance with the present invention I duction of mica articles that adhesives having produce improved electrically insulating the particular and peculiar property of wetting terials by bonding together sup rposed mica g0 and effectively bonding together mica flakes are flakes with a cementing agent comprising a 20 relatively few in number, probably not more than resinous composition of the class produced by six or eight such adhesives thus far being known. condensing an aldehyde with a product of b The binder .most commonly used, despite its drolysis of a polyvinyl ester. One embodiment known disadvantages, has been a natural resin of my inventionis illustrated in the accompanysuch as shellac. Gum copal also has been em ing drawing, wherein is shown a mica com- 25 ployed, as also certain inorganic compositions mutator cone comprising superimposed mica I containing sodium silicate, or lead borate, or flakes bonded together with the aforementioned compounds of lead, boron and silicon. Certain resinous compositions used in carrying the pressynthetic resinous materials also are capable of cut invention into effect. Such resinous comwetting and cementing together mica fragments. positions are described, generally, in Reissue Pat- 30 One such material is the reaction product of a out 20,430, Morrison et al., and, as pointed out polyhydric alcohol such as glycerine and a polyin said patent, may be produced from various basic acid, or the anhydride thereof, such as aldehydes and various polyvinyl esters. In th phthalic acid or anhydride. Such a reaction following description of the invention and in product is now generally known as an alkyd the appended claims, this class of resins is desig-- 35 resin. Laminated articles comprising superposed nated, generally, as polyvinylal resins". A mica flakes bonded together with an alkyd resin polyvinylal resin may be prepared, for instance, are described in Barringer and Peterson Patent as follows: 1 1,589,094, and the method of making such ar- One hundred parts of a polymerized vinyl ticles, in Peterson Patent 1,619,758, both of which acetate is dissolved in 185 parts of glacial acetic 40 patents are assigned to the same assignee as the acid. To this is added 83 parts of an aqueous present invention. solution of formaldehyde, containing 37 per The known binders heretofore used or sugcent of formaldehyde, and a suitable amount of gested for cementing together mica fragments mineral acid, for example 6.8 parts of concenhave certain disadvantages. For example, when trated sulfuric acid. All proportions herein given 45 shellac-bonded mica is highly heated as an inare by weight. Hydrolysis is carried out at about cident to soldering or similar operations in the 70 C. in an enameled vessel. Samples of the manufacture of electrical devices, the shellac reaction mixture areremoved at suitable intermay sometimes soften suillciently to permit slipvals of time and analyzed for their formaldehyde ping or sliding of adjacent mica flakes. Shellac content. The results of analyses indicate the 50 also hasa tendency to swell when heated. This extent to which the polyvinyl ester has been ,conproperty often causes deformation in mica strucverted into polyvinyl acetal, specifically, polytures bonded with shellac, particularly in cases vinyl formal. When the desired degree of. conwhere part of the mica extends beyond the rest version has been effected, a suitable a'snountof o of the structure of which it forms a part. Fura neutralizing agent, for example 13 parts of concentrated ammonia, is added to the reaction vessel. The neutralized mass is poured as a thin stream into water, whereupon the reaction product is precipitated in the form of threads. The thread-like material is washed with water and dried in a current of warm (60 C.) air. The dried threads are white, or nearly so.

Aldehydes other than formaldehyde may be used in making polyvinylal resins, for example. acetaldehyde, propionic aldehyde, butyric aldehyde, benzaldehyde and the like. Likewise polyvinyl esters other than polyvinyl acetate may be employed, for instance polyvinyl propionate, polyvinyl butyrate and the like. The properties of polyvinylalresins may be varied through a wide range by varying the viscosity and the extent of the hydrolysis of the polyvinyl ester, the amount and the character of the aldehyde reacted with the hydrolyzed polymerized vinyl ester, and the character and the amount of acid catalyst used.

Conveniently the polyvinylal resin, modified, if desired. as hereinafter will be described, is applied in solution state to the surface of the mica flakes and is drawn in between the laps of the mica flakes by capillary force. As a solvent for the polyvinylal resin I may use, for example, furfural, ethylene dichloride, dioxan, chloroform, or a. mixed solvent formed, for instance, of an aromatic hydrocarbon such as benzene, toluene, or xylene and any one or more of the following monatomic saturated alcohols: methyl, ethyl, n-propyl, n-butyl, n-amyl, hexyl or octyl. The percentage by weight of the alcohol in such a solvent mixture may vary, for instance, from '20 to 40 per cent, the remainder being aromatic hydrocarbon. A more specific example of a suitable solvent is one formed, by weight, of approximately 25 per cent ethyl alcohol and approximately per cent toluene.

The resin solution as applied to the mica flakes may contain any suitable amount of polyvinylal resin, but ordinarily will contain from about 5 to 20 per cent by weight ofresin. I have found that a binder formedof-a solution containing approximately 10 to 15" per cent by weight of a polyvinylal resin obtained by condensing formaldehyde with a product of hydrolysis of polyvinyl acetate is particularly adapted for wetting and bonding together superposed mica flakes. Only suflicient solution should be used to cause a complete wetting and cementing together of the mica'fragments. Ordinarily about parts by weight of a resin solution containing approximately 10 to 15 per cent by weight of polyvinylal resin will effectively wet and bond about 75 to 300 parts by weight of mica. Of course it will be understood that the ratio of binder to mica will vary with "such influencing factors'as the particular polyvinylal resin and solvent employed, temperature and technique of application, particle size and source of the mica, and so on. In general, it may be said that the amount of binder, calculated on the basis of the solid resin, is less than is required with shellac and other resinous binders heretofore ing or any other suitable means of applying the binder to the mica fragments may be employed.

The built-up sheets or plates of mica thus pro duced are covered on both sides with dry mica to prevents sticking to the supports and are placed in an evacuated oven heated to a temperature sufllciently high to volatilize the solvent. This temperature may range, for instance, from to 0., depending upon the particular solvent employed. The mass is allowed to remain in the oven for a period of time varying with conditions, but ordinarily will be from about 30 minutes to two hours. During this preliminary heating step the solvent is thoroughly removed.

The mica plates are then placed between cushions in a hydraulic press as described in Peterson Patent 1,098,967. After the temperature has become equalized by turning on the steam used for heating the press for about 15 minutes, the mica plate is compressed under a pressure of about 500 to 1000 pounds per square inch while simultaneously heating the same to a temperature of about 150 to C. .The hydraulic pressure should be released several times ,during the flrst flve to ten minutes to allow the escape of volatile ingredients in the sheets. When no more vapors are being given off, full pressure of about 1000 pounds per square inch is applied continuously to the mica sheets for about 45 minutes.

The sheets thus produced are milled to equalize the thickness. Thereafter they are cut or sawed to produce developed shapes that later can be molded into cones for the commutators of dynamo-electric machines or into other desired articles. 7

After the developed shapes have been preformed they are placed into a mold, and heated to approximately 150 to 200 C., while simultaneously applying pressure to give the articles the desired form. During this molding treatment, which requires only a few minutes, the initially thermoplastic and relatively soft polyvinylal resin is converted to a hard, flexible,

tough, abrasion-resistant state. In this form the resinous binder is resistant to the softening action of solvents such as oils, varnishes, and the like. It has good moisture resistance and high dielecfiic strength. Further, it does not readily I oxidize and become brittle when heated for long periods of time. It softens slightly at high temperature, but not sumciently that it oozes from between the mica flakes and permits sliding of the flakes when under pressure. This softening of the binder at an elevated temperature is particularly advantageous in the assembling of mica cones in commutators, since it is impracticable to form the metal and mica parts of the commutator to exact dimensions. Hence when mica cones, such as shown by way'of illustration in the accompanying drawing, are assembled with the other parts of the commutator and heated to a temperature of about 150 to 200 C. under pressure, the binder will yield sufflciently to cause the mica to assume a snug mechanical flt with respect to the adjoining metal parts without cracking. I

For certain applications it is advantageous to modify the polyvinylal resin by incorporating therewith, prior to use, a suitable proportion, for example up to 50 per cent. preferably from about 5 to 40 per cent, of a potentially reactive synthetic resin. Examples of such resins are those known, generally, as phenol-aldehyde, ureaaldehyde, aniline-aldehyde resins, and the like. A' resinous condensation product of a phenol,

more particularly cresol, and formaldehyde is a preferred modifying agent.

The potentially reactive resin is converted to an insoluble .and infusible state while forming the mica plate under heat and pressure, or during subsequent service use of the material. The presence of such heat-resisting resin in the binder serves to increase the resistance to flow of the polyvinylal resin at the elevated temperatures encountered during use of the insulation in electrical apparatus, without materially afiecting the high dielectric strength and wetting power toward mica of the polyvinylal resin. Other addition materials also may be incorporated in the resinous binder, as desired or as conditions may require.

The mica products of this invention are oiland moisture-resistant, have good are resistance, a high dielectric strength and, in general, can be made with less binder than has been required heretofore for the manufacture of such products. The new products can be made in varying flexibility to meet particular installation requirements, and thus facilitate the construction of I various types of electrical apparatus. Commutator segments and cones made of mica flakes pasted with polyvinylal resin are exceptionally firm and strong. Small motor commutators in which such pasted mica was used were spun hot at high speed. The mica flakes showed no tendency to slip and the resin binder did not ooze from the pasted mass.

Unlike shellac, the polyvinylal resin binder shows no tendency to form harmful decomposition products at temperatures encountered during operation of electrical machinery. The binder is not attacked by such solvent agencies as alcohols, acetone, naphthas and other common solvents as is shellac. In brief, the polyvinylal resin fulfills all the exacting conditions requisite for a bonding agent for mica flakes from the standpoint of both physical and electrical properties. As a result, laminated mica compositions of superior quality and a wide field of utility are produced.

What I claim as new and desire to secure b Letters Patent of the United States, is:

1. An electrically insulating material comprising fragments of mica bonded together with a cementing agent comprising a poEyvinylal resin.

2. A laminated insulating composition consisting of flakes of mica superposed upon each other and bound together with a cementing agent comprising a polyvinylal resin modified with a potentially reactive resin converted to the insoluble and infusible state by heating under pressure.

3. An electrically insulating material comprising fragments of mica superposed upon each other and cemented together with a binding medium comprising a polyvinylal resin obtained by condensing formaldehyde with a product of hydrolysis of polyvinyl acetate.

4. A laminated article consisting substantially of flakes of mica bonded together with a cementing agent comprising a polyvinylal resin modified with a phenolaldehyde resin in an insoluble and iniusible state.

5. An electrically insulating material consisting ot'fragments of mica bonded together with a cementing agent comprising a polyvinylal resin modified with from 5 to 40 per cent by weight thereof of a phenol-formaldehyde resin in an insoluble and infusible state, said polyvinylal resin being obtained by condensing formaldehyde with a product of hydrolysis of polyvinyl ester.

W'ILHAM HOWARD MILLER. 

